Multichannel Adaptive Polarization Mode Dispersion Compensation with One Compensator in Dense Wavelength Division Multiplexing Transmission Systems

A simple two-section polarization mode dispersion(PMD) compensator is proposed for multichannel PMD compensation, which can compensate two or even more channels simultaneously. Because of the statistical characteristics and the frequency-dependence of PMD, for current single mode fiber with moderate PMD, the probability that all channels are severely degraded at the same time is extremely small, which makes it possible to compensate a dense wavelength division multiplexing(DWDM) transmission system with moderate PMD using this compensator. It is shown that the outage probability of a 40×43 Gb/s DWDM transmission system using this compensator is decreased significantly from 3.6×10-3 to 3.6×10-5.     

Tracking and separation of time-varying principal states of polarization in optical fiber link for first-order PMD compensation and its filter-dependent performance

We propose a simple method for tracking and separating time-varying principal states of polarization (PSP) occurring in a fiber-optic transmission link with polarization mode dispersion (PMD) for use in PMD compensation. The proposed method uses as a feedback monitoring signal the bandpass-filtered RF power at bit-rate frequency for NRZ signal format. We demonstrated the operating principle of the method through theoretical simulation and experiment using an automatically adaptive PMD compensator employing a single polarization beam splitter (PBS). The effects of electrical filtering on the PSP tracking performance were also investigated by using three types of filters, i.e., a low-pass filter (LPF), a band-pass filter (BPF), and a high-pass filter (HPF). The simulation results showed that only a BPF centered at bit-rate frequency was found to allow for tracing and separation of two PSPs via PBS. The proposed method, when applied to a conventional PMD compensator that alternately controls a polarization controller and a delay line, enables separation of PSP control from differential-group-delay (DGD) control, thus allowing fast tracking of rapidly changing PSP in a PMD-impaired optical fiber link and reducing compensation time.     

Adaptive PMD compensation experiment in 40 Gb/s CSRZ system

In 40 Gb/s systems with low polarization-mode dispersion(PMD) fibers,first-order PMD is the dominate factor with quite small value. An adaptive PMD compensator consisting of an electrical polarization controller(PC) ,a section of polariza-tion maintaining fiber(PMF) ,a degree of polarization(DOP) detector and a feedback control module is employed in 40 Gb/s NSFCNet. This compensator has a simple structure and high speed,which can compensate up to 20 ps first-order PMD adaptively and the average searching time is 2 ms.     

40 Gb/s CSRZ码系统下的自适应PMD补偿实验

在采用了新型光纤的40Gb／s系统中，通常只有很小的一阶偏振模色散（PMD）。本文在40Gb/s的国家自然科学基金网（NSFCNET）上，搭建了由电控偏振控制器（PC）、保偏光纤（PMF）、信号光偏振度（DOP）检测单元以及反馈控制单元组成的PMD补偿模块，具有结构简单、反应灵敏的特点，可以对小于20ps的一阶PMD进行自适应的补偿，平均搜索时间为2ms。     

偏振模色散的分析模型

当光纤通信系统单信道速率到40Gbit/s以上时，偏振模色散的影响已经成为严重影响系统性能的主要因素。对偏振模色散现象的研究通常可采用几种不同的分析模型：琼斯矩阵、斯托克斯空间、耦合非线性薛定谔方程；对这几种分析模型作了详细地介绍，进行了分析和比较，并对40Gbit／s光纤传输系统进行了数值模拟。     

Large PMD Compensated by Four Free Degrees in OTDM System

By introducing a two-stage polarization mode dispersion (PMD) compensator after a optical fiber link with a large PMD,over 270 ps first-order and 2 000 ps2 high-order PMD was compensated. The results show that the two-stage compensator can be used to PMD compensation in the 20 Gb/s optical time division multiplexing system with 60 km high PMD fiber. After compensating, the 270 ps DGD is changed into max. 7 ps. Moreover,the tunable FBG has a function of dispersion compensation.     

Practical considerations for optical polarization-mode dispersion compensators

For transmission systems at 10 Gb/s and beyond, polarization-mode dispersion (PMD) is one of the limiting factors. Optical PMD compensators aim at increasing the PMD value tolerated by the system; however, they do not cancel out its effects. Therefore, the performance of a PMD compensator is assessed statistically. Requirements for optical PMD compensators include a response time in the range of 1 ms in order to follow polarization fluctuations over the line. The system design should account for the interaction of other transmission impairments with the PMD compensator operation. For instance, transmitter chirp and residual chromatic dispersion have a deleterious impact on the compensator performance. While self-phase modulation is harmless, cross-phase modulation greatly reduces the compensator efficiency. System design rules have been applied to a one-year field trial, showing the compensator's efficiency and reliability. However, reducing their cost is the next challenge that will bring optical PMD compensators to be used in installed systems.     

2.5 Gb/s偏振模色散自动补偿对偏振模色散容限值的影响

在我国现有的光纤通信骨干网中,绝大部分速率为2.5 Gb/s。今后如在我国现有的2.5 Gb/s网络系统上进行密集波分复用(DWDM)升级,亦需要考虑偏振模色散(PMD)对系统容量升级的影响。采用十段高双折射光纤级联而成的偏振模色散模拟器模拟实际光纤,从信号中提取基带频率分量作为反馈信号,对2.5 Gb/s系统进行了偏振模色散自动补偿实验,并对反馈前后的系统进行了系统代价的测量和比较。实验结果表明,偏振模色散自动补偿能较大幅度地提高系统的偏振模色散容限值。     

Dynamic PMD compensator for 40 Gb/s PM-DQPSK system

The adaptive polarization mode dispersion(PMD) compensation in high-speed transmission system has become more and more necessary for the link PMD causing strong signal distortions.A dynamic adaptive PMD compensator in 40 Gb/s polar-multiplex differential quadrature phase shift keying(PM-DQPSK) system is reported.Experimental results show that the PMD compensator can track the average polarization state variation at 65 rad/s without any lost of the optimum tracking.The 1st-order PMD compensation is demonstrated experimentally,and the compensator can increase the maximal tolerable PMD value by 26 ps from 17 ps to 43 ps in an optical transmission system.     

对40 Gb/s OTDM系统中光纤光栅色散补偿器进行偏振模色散补偿的研究

改变光纤光栅紫外曝光系统 ,在相位掩模板后插入一个旋转装置 ,使得光纤在制作过程中可以进行某种旋转。通过这种方法制作的光纤光栅偏振模色散减小到平均差分群时延 (DGD)约为 0 2ps,而没加旋转制作的光纤光栅平均DGD约为 18 2 ps。采用两个这种低偏振模色散 (PMD)的光纤布拉格光栅 (FBG) ,成功地在 4 0Gb/s光时分复用 (OTDM)系统中补偿了约 2 0 4 0 ps的色散 ,该系统在经过 12 2km普通单模光纤传输后 ,未发现PMD的影响 ,传输功率代价小于 1 4dB。     

On the second-order approximation of PMD

A second-order polarization mode dispersion (PMD) approximation based upon the pulse-width distortion has been studied. It shows that a complete second-order approximation should include the second derivative of the PR-ID vector as well as the first derivative of the PMD vector. Second-order pulse distortions are explicitly expressed including a `first-order' term involving principal states of polarization (PSP) of the pulse and a second-order term involving the beating between fiber chromatic dispersion and effective PMD chromatic dispersion. An analytical result is derived for the probability of second-order PR-ID power penalty. It shows that the mean PMD of the fiber should be restricted to 26 ps and 18 ps, respectively for an optical link with zero and 850 ps/nm chromatic dispersion, in order to maintain a one dB second-order PMD power penalty with a probability below 10^-6 at a data rate of 10 Gb/s. The analysis also indicates that a second-order PMD compensator can be used as a dynamic chromatic dispersion compensator     

Multi-thousand kilometer optical soliton data transmissionexperiments at 5 Gb/s using an electroabsorption modulator pulsegenerator

We have demonstrated the transmission of data-coded optical soliton pulse at the bit-rate of 5 Gb/s over a few thousand kilometers. Ultra-short optical pulses generated with a sinusoidally driven electroabsorption modulator and a DFB-LD that were approximately hyperbolic-secant squared shape were transmitted through the fiber. Both recirculating loop transmission lines and straight transmission lines were used for the experiments. The pulse width was maintained well over 6,000 km transmission in the recirculating loop experiment. Three-thousand-km stable transmission was achieved using a straight-line setup at the bit-rate of 5 Gb/s, although the average fiber chromatic dispersion deviated slightly from the optimum value. The results indicate the possibility of the soliton transmission system for multi-thousand-km optical communication systems     

Automatic compensation of polarization-mode dispersion for 40 Gb/s transmission systems

We present an automatic compensator that effectively mitigates signal distortion due to polarization-mode dispersion (PMD). Accurate compensation is achieved by utilizing a degree of polarization (DOP) monitor with a measurement uncertainty of better than 1% achieved by applying a self-dependent precalibration procedure. The compensation performance at 43 Gb/s was evaluated systematically with respect to both first- and second-order PMD by using a crystal-optical PMD emulator. The compensator extended the tolerable differential group-delay (DGD) limit from 8 to 28 ps, while maintaining the Q penalty below 1 dB. In terms of the average of a Maxwellian-distributed DGD, the PMD compensator enabled transmission for up to 8 ps, or about three times higher than the level tolerated in the uncompensated case. In addition to the compensation performance, we demonstrate successful operation, even for distorted signals with high chromatic dispersion. We believe this capability will be a key enabler for a combined operation with adaptive chromatic dispersion compensators.     

A Change of Perspective on Single- and Double-Stage Optical PMD Compensation

We present a rigorous investigation on how to optimize the degrees of freedom of optical polarization mode dispersion (PMD) compensators composed of differential group delay sections and polarization controllers, up to two stages. The analytical treatment relies on the extracted Jones matrices of the transmission and compensation fibers. The analysis of a single-stage compensator with two degrees of freedom (fixed DGD) is based on the maximization of the eye opening, as provided by the generalized Chen formula. The outage probability is quantified through a fast semi-analytical technique. It is shown how the benefits of single-stage compensation are strongly reduced and can lead to outage events, when certain critical input states of polarization are launched into transmission fibers with strong eigenmodes depolarization (i.e., strong higher order PMD). Focusing on such transmission fibers and input configurations, a novel algorithm is introduced for controlling a double-stage compensator with five degrees of freedom. The algorithm is based on an ideal equalization of the transmission fiber at half the bit-rate, realized resorting to spherical geometry. To this aim, we show that the first compensator stage must be a PMF fiber with very large DGD, equal to the bit period, in order to compensate the most critical configurations associated with outage events.      

An optical bit-rate flexible transmission system with 5-Tb/s-kmcapacity employing multiple in-line erbium-doped fiber amplifiers

An optical bit-rate flexible transmission system using erbium-doped fiber amplifiers (EDFAs) is proposed, and the system design is discussed. An optical bit-rate flexible system using multiple in-line erbium-doped fiber amplifiers has produced a regenerative repeater spacing of 505 km at 10 Gb/s and 523 km at 5 Gb/s for direct-detection systems. This system proves that an optical bit-rate flexible system with a transmission capacity of 5.05 Tb/s-km can be feasibly constructed. System capacity is clarified both theoretically and experimentally. The power penalties involved are discussed. The related optical and electrical circuits proved operational above 10 Gb/s. The 523 km at 5 Gb/s and 505 km at 10 Gb/s transmission experiments successfully demonstrated that EDFAs effectively enhance a system's transmission capacity     

基于偏振复用的WDM-OFDM-PON

波分复用无源光网络(WDM-PON)是一种容量大、易升级、网络安全性高的光接入网技术。利用正交频分复用(OFDM)调制技术在传输速率、距离和色散容限方面的优势,提出一种基于偏振复用的WDM-OFDM-PON,同时在光网络单元(ONU)中实现了无光源的无色设计。仿真实验表明该系统可以在50km单模光纤上利用每一波长实现下行10Gb/s、上行5Gb/s的双向传输。     

System performance evaluation of the FSA submarine opticalamplifier system

The FSA submarine optical amplifier system developed for commercial use is designed to transmit 2.5 and 10 Gb/s signals flexibly; its repeater spacing is 90 km. It contains six line pairs to yield a maximum transmission capacity of 60 Gb/s. Its system configuration, and the characteristics of its fibers and optical amplifiers, which realize effective dispersion and optical passband management, are introduced. We discuss its performance with regard to the parameters significant in optically amplified transmission: evolution of zero dispersion wavelength (ZDW), polarization mode dispersion (PMD), and fiber nonlinearity induced impairments. Signal-to-noise ratios (SNR's) for 2.5 and 10 Gb/s signal transmission are measured and the improvements offered by polarization scrambling are also discussed. Finally, ZDW, PMD, and SNR characteristics of the system after installation are reported     

MLSE Equalizers for Frequency Discrimination Receiver of MSK Optical Transmission System

We propose maximum-likelihood sequence estimator (MLSE) equalizers based on either Viterbi algorithm or template matching temple matching (TM) for the equalization of impairments imposed on the minimum shift keying (MSK) modulation formats in long haul transmission without optical dispersion compensation. The TM-MLSE equalizer is proposed as a simplified alternative for the Viterbi-MLSE equalizer. It is verified that the Viterbi-MLSE equalizer can operate optimally when noise approaches a Gaussian distribution. Simulation results of the performances of the two MLSE equalizers for optical frequency discrimination receiver-based optical MSK systems are described. The transmission performance is evaluated in terms of: (1) the chromatic dispersion (CD) tolerance for both Viterbi-MLSE and TM-MLSE equalizers; (2) transmission distance limits of Viterbi-MLSE equalizers with various number of states; (3)the robustness to fiber polarization mode dispersion (PMD) of Viterbi-MLSE equalizers; and (4) performance improvements for Viterbi-MLSE equalizers when utilizing sampling schemes with two and four samples per bit over the conventional single sample per bit. With a small number of states (64 states), the non-compensating optical link can equivalently reach up to approximately 928 km SSMF for 10 Gb/s transmission or 58 km SSMF for 40 Gb/s. The performance of 16-state Viterbi-MLSE equalizers for optical frequency discrimination receiver (OFDR)-based optical MSK transmission systems for PMD mitigation is also numerically investigated. The performance of Viterbi-MLSE equalizers can be further improved by using the sampling schemes with multiple samples per bit compared to the conventional single sample bit. The equalizer also offers high robustness to fiber PMD impairment.     

Polarization mode dispersion compensation at 10, 20, and 40 Gb/swith various optical equalizers

Polarization mode dispersion (PMD), especially in “old” fibers, is considered harmful for installation and upgrading of trunk lines. An optical PMD equalizer should have several or many differential group delay (DGD) sections with polarization transformers in between which can endlessly transform any input polarization into a principal state of the following DGD section. The sections must practically have fixed DGDs unless there is only one section. The small-signal baseband transfer function for PMD, higher order PMD, and the necessary number of sections as well as their control by the output signals of an electrical filter bank in the receiver are also discussed in this context. Several PMD equalizers have been realized and successfully tested in transmission systems with bit rates of 10, 20, and 40 Gb/s. The systems operated stably with well-opened eye diagrams for DGDs ranging between 0 and 1.7 bit durations. Best performance is obtained from a distributed PMD equalizer with one piece of polarization-maintaining fiber twisted by 64 stepper motors. The principle can also be realized in LiNbO₃     

Deterministic approach to first- and second-order PMD compensation

The authors show that three concatenated first-order polarization mode dispersion (PMD) segments are sufficient to compensate any first- and second-order PMD present in a transmission fiber. They determine analytically the required individual rotation matrix of the polarization rotators and the required differential group delay for the variable delay line in the compensator.